1. Field of the Invention
The present invention relates to a filter for collecting particulate materials included in an exhaust gas of an engine, especially a car engine.
2. Description of Related Art
A diesel engine has a good thermal efficiency as compared with a gasoline engine, and is advantageous as an engine for a car which meets demand for a decrease of CO2 emissions as a global warming countermeasure. On the other hand, however, there has been the problem of generation of particulate materials (particulate matter (PM)) by diffusion burning.
This PM is mainly constituted of carbon fine particles of soot and the like, and it is recognized that the PM has carcinogenicity. Therefore, it is necessary to prevent the PM included in an exhaust gas of the car engine from being discharged to the atmospheric air, and strict emission regulations are imposed on the PM. Especially in recent years, the strictness of the PM emission regulations has increased remarkably, and in European Union (EU) and the like, emission regulations on the basis of the number of particulates of the PM are being introduced in addition to conventional emission regulations on the basis of a mass of the PM.
To cope with such strict emission regulations, many investigations have been conducted to decrease an amount of the PM emissions, but there is a limit to the decrease of the amount of the PM emissions by improvement of a burning technology. Therefore, at present, a method of disposing a filter in an exhaust system has been the only effective means for decreasing the amount of the PM emissions.
As the filter for collecting the PM, a wall flow type filter using a honeycomb structure has broadly been used, because a high PM collecting efficiency can be obtained while suppressing a pressure loss in an allowable range. This honeycomb structure has porous partition walls to define and form a plurality of cells extending from an inlet end surface that becomes an inlet side of an exhaust gas to an outlet end surface that becomes an outlet side of the exhaust gas, and plugging portions to plug open ends on the side of the outlet end surface of the predetermined cells and open ends on the side of the inlet end surface of the remaining cells.
In the filter using such a honeycomb structure, the exhaust gas allowed to flow from the inlet end surface into the cells passes the partition walls, and then flows out from the outlet end surface to the outsides of the cells. When the exhaust gas passes the partition walls, the partition walls function as filter layers, so that the PM included in the exhaust gas is collected.
Additionally, for using such a filter continuously for a long period of time, it is necessary to periodically subject the filter to a recovery treatment. That is, for decreasing the pressure loss increased due to the PM deposited in the filter with an elapse of the time to return a filter performance to an initial state, it is necessary to remove the PM deposited in the filter by burning the PM with a high temperature gas. Moreover, during this recovery, a heat stress is generated in the filter by heat of the burning of the PM, and hence the filter is damaged sometimes.
Heretofore, as a countermeasure for preventing such damage of the filter, it has been suggested that the whole filter is not manufactured as one honeycomb structure, but the honeycomb structure for the filter is manufactured by joining a plurality of segments having a honeycomb form (the honeycomb segments). Specifically, a segment structure joined and integrated by an easily deformable joining material having a low elastic modulus is provided between the plurality of honeycomb segments, to disperse and alleviate the heat stress which acts on the honeycomb structure during the recovery, whereby enhancement of a heat shock resistance is achieved (e.g., see Patent Document 1).
However, in the outlet end surface where the heat stress during the recovery is severest, an excessively large temperature rise occurs locally, though at very low frequency. Therefore, even in the filter using the honeycomb structure of the segment structure, the heat stress alleviating effect is not sufficiently obtained sometimes. In consequence, a low temperature portion is pulled due to thermal expansion of a high temperature portion in which the excessively large temperature rise has occurred, and a tensile stress is generated in the low temperature portion. As a result, there has been the problem that cracks are generated in this low temperature portion.
It is to be noted that from the viewpoint that the honeycomb structure is prevented from being damaged due to the local temperature rise of the honeycomb structure during the recovery of the filter, a material having a large heat capacity is preferably used. Therefore, it has been suggested that as a material of the honeycomb structure constituting the filter, a material containing silicon carbide as a main component is used. However, a thermal expansion coefficient of silicon carbide is excessively large, and hence there has been the problem that the damage due to the heat stress (heat stress cracks) is caused.
Moreover, a method has been also suggested in which a porosity of the material of the honeycomb structure for use in the filter is lowered to increase the heat capacity, and the temperature rise is suppressed to prevent the heat stress cracks. However, in this method, as a result of the lowered porosity, there has been the problem that pores of the partition walls that become the filter layers are easily clogged with the collected PM, and the pressure loss increases.
Furthermore, another method has been suggested in which constituent particles (an aggregate) of the material of the honeycomb structure for use in the filter are increased, to enhance a thermal conductivity and to suppress the temperature rise, whereby the heat stress cracks are suppressed. However, in this method, the pores get excessively large, and hence there has been the problem that a PM collecting efficiency deteriorates.
Additionally, a further method has been contrived in which the pores of the honeycomb structure for use in the filter are increased in size and Young's modulus of the material is lowered, to increase a strength/Young's modulus ratio and to heighten a limitation of strain which reaches the heat stress cracks, whereby the heat stress cracks are prevented. However, also in this method, as a result of the increased pores, there has been the problem that the PM collecting efficiency deteriorates.
[Patent Document 1] JP-A-2002-253916